SEA URCHINS 69 



stomach and intestine, but is coiled spirally around the interior 

 of the test, to which it is attached by a mesentery. The water- 

 vascular system presents certain modifications and anatomical differ- 

 ences from that of the common starfish, but, as its function is identi- 

 cal, a detailed description of its component parts is unnecessary. 



The Common Sea Urchin (Echinus csculentus) may be dredged 

 up all round the English coast, and in such sheltered inlets as the 

 Clyde it may often be found stranded on the shore at low-tide 

 mark, or in the pools. Wherever the coast is exposed, however, 

 it will only be obtainable with the dredge from depths of 20 or 

 more fathoms, where it is beyond the depth of wave disturbance. 

 It may attain to the size of a very large orange, flattened on its 

 under surface, tapering slightly towards the aboral pole. It is 

 covered with spines, which are short in comparison with the dia- 

 meter of the body, and are usually whitish, sometimes tipped with 

 purple. The body is generally a brownish red or purplish 

 colour. The Purple Egg-Urchin (Echinus miliaris) has long, 

 purple spines, never attains to the size of the Common Urchin, and 

 is frequently to be found in holes in the rocks projecting into 

 the sea so as to form reefs and ledges accessible at low water. 

 Whether these refuges are actually excavated, as has been some- 

 what hastily and dogmatically stated by some authors, would 

 appear doubtful, and is a matter requiring more careful and accu- 

 rate observation than it has yet received. As Mr. E. W. L. Holt, 

 Scientific Adviser to the Irish Board of Fisheries, points out, one 

 side of the cavity is not infrequently formed by calcareous algae, in 

 which instances it would appear as if the Urchin had wandered 

 into the cavity and become imprisoned by the growth of the algae. 



The Clypeastrida, 1 or Cake Urchins, are all extremely flattened 

 in form, and have the anus (periproct) shifted from the apical pole 

 to near the margin of the dorsal surface, so that it is no longer 

 surrounded by the genital plates ; and some of the tube-feet of 

 the dorsal surface are flattened and modified into respiratory 

 organs. They are all more or less sand dwellers, living half buried 

 in the sand, or moving along its surface by the aid of their small 

 locomotor tube-feet. All are more or less oval or circular in out- 

 line, and resemble, as may be guessed from their popular name, 

 little flattish cakes. 



1 Latin, clypeus, a shield ; Greek, aster, a star ; eidos, form. 



